step1：新建ViewPoint类

该类用于存放点坐标,就是手指接触ImageView并移动时的坐标

package com.example.handwritingimageview;

public class ViewPoint {
    float x;
    float y;
}

step2：新建Line类

顾名思义，该类存放点的集合，即一条线

//表示一条线
public class Line {
    ArrayList<ViewPoint> points = new ArrayList<>();
}

step3：编写HandWritingImageView类

该类继承AppCompatImageView，先重写父类构造器，再重写onTouchEvent和onDraw方法,在onTouchEvent方法中多次使用了 invalidate()这个方法，这个方法是用来触发onDraw方方法进行画图。

package com.example.handwritingimageview;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.util.Log;
import android.view.MotionEvent;
import android.widget.ImageView;


import androidx.appcompat.widget.AppCompatImageView;
import androidx.core.widget.ImageViewCompat;

import java.util.ArrayList;


public class HandWritingImageView extends AppCompatImageView {
    private Paint paint = new Paint();

    //当前正在画的线
    private Line current = new Line();

    public HandWritingImageView(Context context) {
        super(context);
    }

    public HandWritingImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public HandWritingImageView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    //画过的所有线
    private ArrayList<Line> lines = new ArrayList<Line>();

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        //获取坐标
        float clickX = event.getX();
        float clickY = event.getY();

        if (event.getAction() == MotionEvent.ACTION_DOWN) {
            //刷新屏幕
            invalidate();
            return true;
        }else if (event.getAction() == MotionEvent.ACTION_MOVE){
            ViewPoint point = new ViewPoint();
            point.x = clickX;
            point.y = clickY;
            Log.d("444", "clickX: "+clickX);
            Log.d("444", "clickY: "+clickY);
            //在移动时添加所有经过的点
            current.points.add(point);
            Log.d("222", "current.points.size(): "+current.points.size());

            invalidate();
            return true;
        }else if (event.getAction() == MotionEvent.ACTION_UP){
            //添加画过的线
            lines.add(current);
            Log.d("222", "lines.size(): "+lines.size());
            current = new Line();
            invalidate();
        }
        return super.onTouchEvent(event);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        //画出之前所有的线
        for (int i = 0; i < lines.size(); i++)
        {
            drawLine(canvas, lines.get(i));
        }

        //画出当前的线
        drawLine(canvas, current);
    }

    private void drawLine(Canvas canvas, Line line)
    {
        for(int i = 0; i < line.points.size()-1; i++){
            float x = line.points.get(i).x;
            float y = line.points.get(i).y;
            float nextX = line.points.get(i+1).x;
            float nextY = line.points.get(i+1).y;
            canvas.drawLine(x,y,nextX,nextY,paint);
        }
    }
}

step4:编写xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <com.example.handwritingimageview.HandWritingImageView
        android:id="@+id/im_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>

效果图

在这里插入图片描述